In the related art within industrial plant environments, particularly those utilizing Industrial Automation (IA), there are control systems for the monitoring and adjustment of control elements or field devices. The control elements or field devices may be used for measuring or adjusting physical properties, quantities, or characteristics of a process, such as flow rate, temperature, level, or pressure.
IA operations must optimize control of the control elements or field devices in order to maximize profitability, while at the same time maintaining safe operation. Control stabilization is key to achieving these goals, and stabilization can often be improved through closer evaluation of a plant's control loops.
In the related art, a control loop may be utilized in order to monitor or adjust a process. Generally, control loops are categorized into open loops, closed loops, and hybrid systems. In the related art, as shown in FIG. 1, a system (101) may attempt to adjust a process (104) through a closed loop system. The process (104) may be a parameter that needs to be controlled, such as flow, level, temperature, or pressure. The output parameter that is controlled or adjusted is called the process variable (PV). The PV is the measured state of the process. A sensor (105) measures the PV, and this measured PV is provided as feedback to the system (101) for the controller (102) to make adjustments in a closed loop system.
In the closed loop system, a set point value (SV) is the desired state of the process. A controller (102) can determine a controller output (OP) sent to a final control element (103) or field device for controlling the final control element (103) to achieve the set point value. The output of the final control element (103) is a positioner signal that is a manipulated variable (MV). Manipulated variables are input variables in process control. The manipulated variables are the input that is controlled by process operator or control system. The manipulated variables are adjusted by the process operator (or control system) to keep controlled variables in the system at constant settings or at their set-points. The manipulated variable (MV) then has an effect on the process (104). The process variable (PV) that is output by the process (104) as a result of the control system is the result.
As the process variable (PV) may not match with the desired set point value (SV), the close loop system controller (102) can compare the data to determine the difference, or error, between the measured process variable (PV) and the desired set point value (SV). Based on the difference, or error, a desired controller output (OP) can be computed to further adjust the final control element (103) in an attempt to lessen the difference between the process variable (PV) and the desired set point value (SV). The controller output (OP) controls movement in the final control element (103), such as a valve. The output of the valve, or the valve positioner signal, then adjusts the manipulated variable (MV), which, in turn, affects the process variable (PV).
In real world applications, there is an issue of stiction for final control elements. Valve stiction exists when resistance prevents movement in a control operation for a valve opening. As a result, additional force is required to move the valve. The application of the additional force causes a sudden movement in the valve.
In the related art, diagnostics related to stiction relate to consideration of one or more of the variables.
U.S. Pat. No. 8,145,328B2 (Patent Document 1) discloses detection and quantification of stiction. According to the related art, consideration of the process variable (PV), controller output (OP), and set point value are contemplated.
US20150112639A1 (Patent Document 2) discloses a method and apparatus for performing diagnostics on a conventional control valve. According to the related art, consideration of the process variable (PV) and controller output (OP) are contemplated.
U.S. Pat. No. 7,274,995B2 (Patent Document 3) discloses an apparatus and method for identifying possible defect indicators for a valve. According to the related art, consideration of the process variable (PV) and controller output (OP) are contemplated.
US20150032270A1 (Patent Document 4) discloses a spectral analysis based detector for a control valve. According to the related art, there is a method using pattern matching the frequency spectrum.